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Joseph Musgrave, political director of the Out4Marriage campaign, said: “Although it’s great to have such a well-known celebrity support us, Hugh’s key contribution is to emphasise the human element.

“This is all about the recognition of love and freeing people to decide how they, and not the state, choose to act on that.”

Grant, 52, best known for his role in Four Weddings and a Funeral, has had a colourful love life.

He and Liz Hurley were together for 13 years - briefly interrupted by his arrest for picking up prostitute Divine Brown on Sunset Boulevard - before he moved on to a three-year relationship with Jemima Khan.

The actor became a father in November 2011, news he announced to the world by releasing a statement saying the pregnancy was unplanned and the little girl was the product of a “fleeting affair” with a woman called Tinglan Hong.

Grant, now a crusader for privacy, appeared on The Ellen DeGeneres Show earlier this year and told viewers that his daughter’s Chinese name, Jingxi, means “happy surprise”.